[Sema] Issue a warning for integer overflow in struct initializer

Clang wasn't issuing a warning when compiling the following code:

struct s {
  unsigned x;
} s = {
  .x = 4 * 1024 * 1024 * 1024
};

rdar://problem/23399683

Differential Revision: http://reviews.llvm.org/D15097

llvm-svn: 257357